1. Field of the Invention
The present invention relates to a semiconductor acceleration detecting apparatus and, more particularly, to a semiconductor acceleration detecting apparatus suitable for use in, for example, vehicles and industrial robots.
2. Description Of The Related Art
FIG. 4 is a cross-sectional and plan view of a conventional semiconductor acceleration detecting apparatus. FIG. 5 is a plan view of the conventional semiconductor acceleration detecting apparatus from which a cap has been removed. The conventional semiconductor acceleration detecting apparatus includes a base 9 which forms, together with a cap 9a, the exterior of the apparatus, and an acceleration detecting beam 2 fixed to the base 9 through a pedestal. The acceleration detection beam 2 includes a fixed portion 6 fixed to the base 9 by means of the pedestal 3, a diaphragm portion 4 formed at one end of the fixed portion 6 integrally therewith, and a movable portion 5 connected to the fixed portion 6 through the diaphragm portion 4. The diaphragm portion 4 has a thickness smaller than the thickness of the fixed portion 6 and movable portion 5, and flexes in response to the acceleration applied to the acceleration detection beam 2. A sensor circuit 1 for generating an electrical signal representing the amount of flexing of the diaphragm portion 4 is formed on the surface of the diaphragm portion 4. The sensor circuit 1 is a bridge circuit of gauge resistor elements 1a which utilize the piezo resistance effect and whose resistance varies according to a stress applied thereto. The electric signal from the sensor circuit 1 is amplified by an amplification circuit 7 disposed on the fixed portion 6 of the acceleration detecting beam 2, and the amplified signal is supplied to an external circuit via wires 11 and lead pins 10. On the fixed portion 6 of the acceleration detecting beam 2 is also disposed a diagnosis circuit 8 for determining whether the signal from the amplification circuit 7 is abnormal. The signal from the diagnosis circuit 8 is also supplied to the external circuit via the wires 11 and the lead pins 10. For the protection of the parts on the base 9, the base 9 is covered with the cap 9a which partially constitutes the exterior of the apparatus.
When an acceleration is applied to the thus-arranged conventional semiconductor acceleration detecting apparatus, a stress concentrates on the diaphragm portion 4 in the acceleration detecting beam 2 and the semiconductor acceleration detecting apparatus thereby flexes at the diaphragm portion 4 thereof. This deformation of the diaphragm portion 4 changes the resistance of the gauge resistors 1a. Accordingly, the bridge circuit is unbalanced and thereby generates an electrical signal which is detected as an acceleration. The generated electrical signal is amplified by the amplification circuit 7, and the amplified signal is supplied outside of the apparatus constituted by the base 9 and the cap 9a via the wires 11 and the lead pins 10.
In the conventional semiconductor acceleration detecting apparatus, the amplification circuit 7 and the diagnosis circuit 8 are disposed on the fixed portion 6 of the acceleration detecting beam 2, as mentioned above, and the fixed portion 6 must therefore have a size large enough to carry the circuits 7 and 8. In other words, the size of the fixed portion 6 cannot be reduced. In addition, since the size of the movable portion 5 greatly affects the characteristics of the acceleration detection apparatus, such as the sensitivity thereof, it cannot be changed carelessly. Thus, the arrangement of the semiconductor acceleration detecting apparatus must be such that a sufficient size is secured in the fixed portion without reducing the size of the movable portion 5. In such a semiconductor acceleration detecting apparatus, however, the size of the acceleration detecting beam 2 is inevitably large and the entire size of the semiconductor acceleration detecting apparatus is, hence, large.